Methane oxidation catalyst, process to prepare the same and method of using the same

ABSTRACT

The invention provides a process for preparing a methane oxidation catalyst, a methane oxidation catalyst thus prepared and a method of oxidizing methane.

FIELD OF INVENTION

The instant invention relates to a methane oxidation catalyst, a processto prepare a methane oxidation catalyst, the methane oxidation catalystprepared by this process and a method of using the methane oxidationcatalysts.

BACKGROUND OF THE INVENTION

One of the most abundant and economically viable alternatives tooil-derived fuels such as gasoline, kerosene and diesel is natural gas.Because of this, engine manufacturers for transport and stationaryapplications are shifting their attention from the traditionaloil-derived fuels to the cheaper, cleaner burning, and moreenvironmentally friendly compressed natural gas (CNG) or liquefiednatural gas (LNG) fuels. In recent years, significant investments andefforts have been made to expand the natural gas fuel supplychain/infrastructure and develop natural gas specific engine hardware inorder to enable the broad deployment of natural gas as s fuel. The majorcomponent in natural gas is methane. The exhaust gas of a natural gasfueled engine may contain some residual level of methane, which has tobe removed before the exhaust gas is released into the atmosphere inorder to meet current and future environmental emission regulations. Oneway of reducing the level of residual methane in the exhaust gas is bycatalytically oxidizing the methane to carbon dioxide and water.Catalysts in current day catalytic converters used to treat exhaust gasare not designed to convert methane. Due to the relatively highactivation temperature required for the combustion of methane, methanewill typically pass through such catalytic converters unconverted.

Catalysts for methane oxidation have been reported previously. InWO2009/057961 a catalyst containing palladium and platinum supported onalumina for treating exhaust gas from a dual fuel, i.e. diesel and LNG,fueled vehicle is disclosed. The catalysts are said to have a preferredpalladium:platinum ratio of 1.0:0.1-0.3 and are deposited on an aluminasupport. However, the performance advantage of thesepalladium-platinum/alumina materials was demonstrated without thepresence of H₂O in the feed. It is well known that exhaust gases fromnatural gas fueled engines in transport and stationary applicationscontain very high levels of H₂O usually in the range of 9-17 vol %.These significant H₂O levels in exhaust gas are known to have a verysignificant adverse effect on the activity of palladium-platinum/aluminacatalysts and the stability of these catalysts in the methane oxidationreaction. Therefore, it is expected that these prior art alumina-basedcatalysts will suffer from excessive activity loss and activity declinerates in commercial applications involving conversion of methane inexhaust gases containing significant levels of water.

U.S. Pat. No. 5,741,467 discloses mixed palladium/alumina andpalladium/ceria/lanthana alumina wash coat formulations used as methaneoxidation catalysts for fuel-lean or fuel-rich methane oxidation,respectively. In addition, U.S. Pat. No. 5,741,467 discloses thatrhodium may be used to substitute completely or in part for thepalladium. However, these catalytic formulations exhibit very lowmethane oxidation activity after aging as illustrated by their hightemperature requirements (higher than 500° C.) for 50 vol % methaneconversion, generally referred to in the art as the T₅₀ (CH₄). The lowmethane oxidation activities and rapid activity declines exhibited bythese catalysts suggest that these catalytic formulations would mostlikely not find acceptance or utility for methane abatement incommercial natural gas-fueled engines exhaust gas treatmentapplications.

US660248 discloses catalysts for removing hydrocarbons from exhaust gascontaining methane and excess of oxygen. The catalysts comprisepalladium or palladium/platinum supported on at least one supportselected from zirconia, sulfated zirconia and tungsten-zirconia. Thedisclosed zirconia-based catalysts show improved performance compared tothe previously discussed alumina-based catalyst with respect to methaneoxidation activity, however the as-reported activity of these catalystsis still too low to be attractive for commercial application.

Therefore, there is a need for methane oxidation catalysts that exhibithigher methane oxidation activity for the efficient removal ofnon-combusted methane from exhaust gas from natural gas-fueled engines.

SUMMARY OF THE INVENTION

It has now been found that a catalyst comprising one or more noblemetals supported on zirconia may show improved methane oxidationperformance, when the zirconia comprises tetragonal and monocliniczirconia in a specific weight ratio range.

Accordingly, the present invention provides a process for preparing amethane oxidation catalyst comprising the following steps:

-   -   a.) calcining a non-modified zirconia precursor at a temperature        in the range of from 675 to 1050° C. to prepare tetragonal        zirconia wherein the weight ratio of tetragonal zirconia to        monoclinic zirconia, if any is present, is greater than 31:1;    -   b.) impregnating the zirconia obtained from step b.) with a        noble metal precursor-comprising impregnation solution;    -   c.) drying the wet, noble metal-impregnated zirconia at a        temperature of no more than 120° C.; and    -   d.) calcining the dried noble metal-impregnated zirconia at a        temperature in the range of from 400 to 650° C. to prepare a        methane oxidation catalyst wherein the weight ratio of        tetragonal zirconia to monoclinic zirconia of the catalyst is in        the range of from 1:1 to 31:1.

The methane oxidation catalyst made by this process provides highermethane oxidation activity, as evidenced by its lower T₅₀ (CH₄)temperatures, as well as better long term hydrothermal stabilitycompared to those methane oxidation catalysts known in the prior art.

In a further aspect, the present invention provides for a method ofoxidizing methane by contacting a gas stream comprising methane with amethane oxidation catalyst according to the invention in the presence ofoxygen and oxidizing at least part of the methane in the gas stream tocarbon dioxide and water.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides methane oxidation catalysts comprisingone or more noble metals supported on zirconia, which zirconia comprisesboth tetragonal zirconia (also referred to as t-ZrO₂) and monocliniczirconia (also referred to as m-ZrO₂).

The zirconia comprising both tetragonal zirconia and monoclinic zirconiaas used in the catalyst of the present invention is not a physicalmixture of tetragonal zirconia and monoclinic zirconia or equally is nota physical mixture of the crystallographic phases of tetragonal zirconiaand monoclinic zirconia. Rather, the zirconia used in the catalyst ofthe present invention was obtained by converting, e.g. thermallyconverting, a noble metal impregnated zirconia material into a methaneoxidation catalyst comprising both tetragonal zirconia and monocliniczirconia resulting in a dispersion of tetragonal and monoclinic zirconiaphases. Such zirconia, comprising both tetragonal zirconia andmonoclinic zirconia, prepared by conversion of a precursor material isherein further also referred to as tm-ZrO₂. It was found that methaneoxidation catalysts comprising noble metals supported on such tm-ZrO₂ ofthe present invention surprisingly exhibit significantly superiormethane oxidation performance, in particular, methane oxidationactivity, compared to prior art methane oxidation catalysts at the samelevels of noble metal loading. Moreover, it was found that methaneoxidation catalysts comprising noble metals supported on such tm-ZrO₂ ofthe present invention also surprisingly exhibit significantly superiormethane oxidation activity compared to methane oxidation catalystscomprising the same level of noble metals supported on a physicalmixture of tetragonal zirconia and monoclinic zirconia, where thetetragonal zirconia and monoclinic zirconia were physically mixed in thesame weight ratio.

The methane oxidation catalyst according to the present inventioncomprises one or more noble metals supported on zirconia. Referenceherein to the term “supported on” is to noble metals that are supportedon the internal and external structure surface of the zirconia,including on the wall surface of any inner pore structure of thezirconia.

As mentioned hereinabove, the zirconia of the present invention on whichthe noble metal is supported comprises at least two crystalline phasesof zirconia, i.e. tetragonal zirconia and monoclinic zirconia. Theweight ratio of tetragonal zirconia to monoclinic zirconia is in therange of from 1:1 to 31:1, preferably in the range of from 2:1 to 31:1,even more preferably 2:1 to 28:1, still more preferably 5:1 to 28:1,even still more preferably the range from 10:1 to 27.5:1 and yet evenmore preferably in the range of from 15:1 to 25:1, yet still even morepreferably, 15:1 to 23:1. It has been found that catalysts comprisingnoble metals supported on a tm-ZrO₂ with the above weight ratio range oftetragonal zirconia to monoclinic zirconia result in an improved methaneoxidation activity, i.e. in a lower T₅₀ (CH₄) value, compared to otherprior art zirconia- or alumina-comprising catalysts for methaneoxidation with the same noble metal loading. The temperature at which atleast 50 vol % of the methane is oxidized is referred to herein as T₅₀(CH₄). The T₅₀ (CH₄) value is a measure for the methane oxidationactivity of a catalyst. A lower T₅₀ (CH₄) value indicates a highermethane oxidation activity of the catalyst. When measured underessentially identical test conditions, the T₅₀ (CH₄) values can be usedto compare the methane oxidation activity of two or more catalysts. Itwas also found that providing a weight ratio of tetragonal zirconia tomonoclinic zirconia in the methane oxidation catalyst according to thepresent invention may result in improved methane oxidation activitystability over prolonged periods of use under hydrothermal conditions.

Herein, the weight ratio of the tetragonal zirconia to monocliniczirconia is the weight ratio as determined by a quantitative XRD phaseanalysis using commercially available software. The so determined weightratio of tetragonal zirconia to monoclinic zirconia in the range from1:1 to 31:1 corresponds to an XRD signal intensity ratio of signalintensity at 20=30.1° (characteristic for tetragonal zirconia) to thesignal intensity at 20=28.1° (characteristic for monoclinic zirconia) inthe range of from 0.8:1 to 12.5:1. A more detailed description of thequantitative XRD phase analysis as used in this invention is providedherein below.

For the purpose of determining the weight ratio of the tetragonalzirconia to monoclinic zirconia, any other zirconia, e.g. a zirconiabinder material that is added to the catalyst by physical mixing is nottaken into consideration.

As mentioned herein above, the zirconia on which the noble metal(s) aresupported comprises tetragonal and monoclinic zirconia. Preferably, themonoclinic zirconia is present as a dispersion of monoclinic zirconia inthe tetragonal zirconia or even as a dispersion of monoclinic zirconiain a (semi-)continuous tetragonal zirconia matrix. Such a structurecould for instance be prepared by the calcination of a single zirconiaprecursor followed by a chemical treatment such as an impregnation stepand further thermal treatment step, i.e., a calcination step. Thezirconia precursor is preferably thermally treated by a calcination stepat a temperature in the range of from 675 to 1050° C. and thenimpregnated with a noble metal solution and thermally treated again attemperatures in the range of from 450 to 650° C.

Without wishing to be bound to any particular theory, it is believedthat such a distribution of tetragonal and monoclinic crystallinezirconia phases cannot be attained by physically mixing tetragonalzirconia with monoclinic zirconia. Furthermore, without wishing to bebound to any particular theory, it is believed that by preparing thezirconia of the catalyst of the present invention, i.e. the tm-ZrO₂,from a single zirconia precursor, the resulting distribution oftetragonal zirconia and monoclinic zirconia in the tm-ZrO₂ allows forthe creation of a high noble metal or metal oxide dispersion on theresulting catalyst. Furthermore, this may provide a catalyst having anadvantageously high noble metal surface area and/or noble metal oxidesurface area. The obtained distribution of tetragonal zirconia andmonoclinic zirconia in the tm-ZrO₂ is believed to restrict thediffusion, migration and/or agglomeration of the noble metal or noblemetal oxide during preparation as well as during use, including the useunder typical hydrothermal methane oxidation operating conditions. Thisbenefits the methane oxidation activity and improves the stability ofthe methane oxidation activity of the catalyst of this invention,including under hydrothermal conditions, such as the conditionstypically encountered in treating exhaust gas from natural gas fueledengines.

As explained herein below in more detail, the zirconia precursor may beany zirconium-comprising compound that converts into zirconia uponexposure to elevated temperatures. The zirconia precursor may alsocomprise, consist or essentially consist of tetragonal zirconia. Withoutwishing to be bound to any particular theory, it is believed that uponexposing the zirconia precursor to elevated temperatures firsttetragonal zirconia is formed, which tetragonal zirconia maysubsequently after impregnation with a noble metal and further thermaltreatment be partially converted to monoclinic zirconia. By preparingthe zirconia of the present invention by one or more thermal,impregnation and further heat treatment steps from a single zirconiaprecursor, a dispersion of monoclinic zirconia in tetragonal zirconiamay be obtained. As mentioned herein above, preferably, the zirconia onwhich the noble metal is supported comprises tetragonal zirconia thatforms a (semi-) continuous matrix structure, with embedded monocliniczirconia in the matrix structure. Suitable zirconia precursors areprovided herein below.

The zirconia on which the noble metal is supported may preferablycomprise a specific surface area in the range of from 10 to 200 m²/g.

The methane oxidation catalyst according to the invention comprises oneor more noble metals. Any combination of two or more noble metals may beused. Preferably, the methane oxidation catalyst comprises one or morenoble metals selected from the group consisting of palladium, platinum,ruthenium, rhodium, osmium and iridium. Preferably, the methaneoxidation catalyst comprises one or more noble metals selected from thegroup consisting of palladium, platinum and rhodium. Preferredcombinations of noble metals comprise: (1) palladium and platinum, (2)palladium and rhodium, and (3) palladium, platinum and rhodium. Suchcombinations of noble metals may provide a methane oxidation catalystwith higher methane oxidation activity, i.e. a lower T₅₀ (CH₄) value anda more stable methane oxidation activity.

The methane oxidation catalyst may comprise in the range of from 0.5 to15 wt % of noble metal, based on the total combined weight of noblemetal(s) and the total weight of the tm-ZrO₂. Preferably, the methaneoxidation catalyst comprises at least 1 wt % of noble metal, based onthe total combined weight of noble metal(s) and the total weight of thetm-ZrO₂.

Typically, the active form of the noble metal capable of catalyzing theoxidation of methane is its metal oxide form. This is for instance truefor palladium. However, in some cases, for example, where both palladiumand platinum are present on the catalyst, a portion of the noble metalwill remain in its metallic form. Therefore, preferably, at least partof noble metal(s) is present in the methane oxidation catalyst in theform of noble metal oxides.

The methane oxidation catalyst may have any suitable shape or formincluding powders, particles, coatings, washcoats, films, extrudates,rings, pellets, tablets or ceramic monoliths of various shapes andsizes. Preferably, the methane oxidation catalyst is provided as aparticle with an average particle size in the range of from 0.1 to 500um, preferably of from 0.5 to 500 μm. The particles may e.g. be in theform of powders or in a washcoat. The methane oxidation catalyst maycomprise zirconia particles that have been shaped to form the abovementioned larger catalyst structures. The methane oxidation catalyst mayfor instance be shaped in a process including extrusion and spraydrying.

The methane oxidation catalyst may optionally contain a binder material.

The methane oxidation catalyst may be deposited on a monolith substratein the form of a coating, washcoat or film. Reference herein to awashcoat is to a dispersion of a material, in this case the methaneoxidation catalyst particles, over the surface area of the substrate,whereby the washcoated material forms a thin layer on the surface of thesubstrate. Suitable substrates include ceramic and metallic monoliths.Such ceramic and metallic monoliths are substrates with nearly uniform,well-defined pore or channel structures. The ceramic and metallicmonoliths may be characterized by the number of pore channels per squareinch; this unit is also referred to in the art as cells per square inchor CPSI. Preferably, the ceramic or metallic monolith substratecomprises in the range of from 50 to 10,000 pore channels per squareinch (323 to 64500 pore channels per cm²), more preferably, 150 to 1000pore channels per square inch (968 to 6450 pore channels per cm²).

In one preferred embodiment, the methane oxidation catalyst of theinvention is provided on a ceramic or metallic monolith substratecomprising pore channels, defining an inner pore channel surface,wherein the methane oxidation catalyst is present in the form of acoating, washcoat or a film of a thickness in the range of from 10 to250 μm on the monolith inner pore channel surface. Preferably, in therange of from 50 to 400 kg, more preferably of from 75 to 300 kg, ofmethane oxidation catalyst per cubic meter of monolith substrate issupported on the monolith substrate. Preferably, the resulting noblemetal content on the monolith is in the range of from 1 to 16 kg/m³ ofmonolith substrate, more preferably in the range of from 1 to 8 kg/m³ ofmonolith substrate.

It is a particular advantage of the methane oxidation catalyst accordingto the invention that it provides higher methane oxidation activity orlower T₅₀ (CH₄) values as well as better long term hydrothermalstability compared to those methane oxidation catalysts known in theprior art.

It is a further particular advantage of the methane oxidation catalystaccording to the invention that it may also show activity toward theconversion of NO to NO₂. This may be of particular value when themethane oxidation catalyst is to be used in an application wherein nextto methane, there is a desire to convert NO (and optionally NO₂) toenvironmentally benign N₂ by using a combination of methane oxidationcatalyst to convert the NO to NO₂ and then a commercially available SCR(selective catalytic reduction) catalyst to reduce NO and/or NO₂ to N₂.This may, for instance, be the case where the exhaust gas to be treatedcomprises methane and NO. An example of such a gas stream would be anexhaust gas from a natural gas fueled engine. The methane oxidationcatalyst may therefore in a particular embodiment of the invention, beprovided in combination with a SCR catalyst, e.g. a SCR catalystcomprising titanium(IV)oxide (TiO₂), tungsten(VI)oxide (WO₃),vanadium(V)oxide (V₂O₅), molybdenum(VI) oxide, noble metals, transitionmetal-exchanged zeolite or a zeolite. The methane oxidation catalystonly needs to convert part of the NO to NO₂ as most SCR catalysts show amore optimal conversion to N₂ where a mixture of NO and NO₂ is providedto the SCR catalyst.

As mentioned herein before, the catalyst of the present invention showsimproved methane oxidation activity, which is evidenced by the lower T₅₀(CH₄) values obtained when oxidizing methane, in particular methanepresent in diluted methane comprising gas streams. In particular, in thecase of the oxidation of methane in a gas stream consisting of less than5000 ppmv of methane and balance of nitrogen, based on the volume of thetotal gas stream, the T₅₀ (CH₄) of the methane oxidation catalyst may beequal to or less than 405° C. for the catalyst according to theinvention. In the particular case, where the weight ratio of tetragonalto monoclinic zirconia in the t/m-ZrO₂ is in the range of from 5:1 to28:1, the T₅₀ (CH₄) value of the methane oxidation catalyst may be equalto or less than 400° C. Furthermore, in the particular case, where theweight ratio of tetragonal to monoclinic zirconia in the t/m-ZrO₂ is inthe range of from 15:1 to 25:1 the T₅₀ (CH₄) value of the methaneoxidation catalyst may be equal to or less than 395° C. More inparticular, in the case of the oxidation of methane in a gas streamhaving a composition consisting of 2000 ppmv CH₄, 1000 ppmv CO, 150 ppmvNO, 7.5 vol % CO₂, 6 vol % O₂, 15 vol % H₂O, and balance of N₂, based onthe volume of the total gas stream, the T₅₀ (CH₄) value of the methaneoxidation catalyst may be equal to or less than 405° C. for the catalystaccording to the invention. In the particular case, where the weightratio of tetragonal to monoclinic zirconia in the t/m-ZrO₂ is in therange of from 5:1 to 28:1 the T₅₀ (CH₄) value of the methane oxidationcatalyst may be equal to or less than 400° C. Furthermore, in theparticular case where the weight ratio of tetragonal to monocliniczirconia in the t/m-ZrO₂ is in the range of from 15:1 to 25:1 the T₅₀(CH₄) value of the methane oxidation catalyst may be equal to or lessthan 395° C.

In another aspect the invention provides a process for preparing amethane oxidation catalyst. The process for preparing a methaneoxidation catalyst comprising the following steps:

-   -   a.) calcining a non-modified zirconia precursor at a temperature        of in the range of from 675 to 1050° C. to prepare tetragonal        zirconia wherein the weight ratio of tetragonal zirconia to        monoclinic zirconia, if any is present, is greater than 31:1;    -   b.) impregnating the zirconia obtained from step b.) with a        noble metal precursor-comprising impregnation solution;    -   c.) drying the wet noble metal-impregnated zirconia at a        temperature of no more than 120° C.; and    -   d.) calcining the dried noble metal-impregnated zirconia at a        temperature of in the range of from 400 to 650° C. to prepare a        methane oxidation catalyst wherein the weight ratio of        tetragonal zirconia to monoclinic zirconia of the catalyst is in        the range of from 1:1 to 31:1.

In step (a) a zirconia precursor may be provided in a calcined form.Preferably, a single zirconia precursor is provided. Any zirconiaprecursor that can be converted into a dispersion of tetragonal andmonoclinic zirconia by the method herein described may be used.

In one embodiment, the zirconia precursor is tetragonal zirconia. Duringthe calcination in step (a) at least part of the tetragonal zirconia maybe converted to monoclinic zirconia. In some instances, small nuclei ofmonoclinic zirconia will be created in the tetragonal matrix during step(a) that may not be discernable via powder XRD measurement. In theseinstances, the calcined zirconia of the step (a) possessing these smallnuclei of monoclinic zirconia could be converted to the desiredtetragonal and monoclinic zirconia during the steps of noble metalimpregnation, drying and calcination. In this later instance, it isbelieved that the impregnation followed by final calcination step areresponsible for the transformation of the zirconia into the desired,beneficial tetragonal and monoclinic zirconia comprising methaneoxidation catalyst.

Alternatively, the zirconia precursor may be amorphous zirconia or azirconium-comprising precursor, including zirconium hydroxide. Withoutwishing to be bound to any particular theory, it is believed that incase of an amorphous zirconia or zirconium-comprising precursor, theamorphous zirconia or zirconium-comprising precursor, during thecalcination of step (a), is initially converted to tetragonal zirconia,and subsequently part of the tetragonal zirconia may be converted tomonoclinic zirconia.

Therefore, the zirconia precursors for the preparation of the tm-ZrO₂ ofthis invention include, but are not limited to, tetragonal zirconia,amorphous zirconia and zirconium-comprising precursors, wherein suitablezirconium-comprising precursors include, but are not limited tozirconium hydroxides and zirconium hydroxide sols, zirconium hydroxidegels, ZrOCl₂, ZrCl₄, ZrO(NO₃)₂, Zr(NO₃)₄, zirconium lactate, zirconiumalkoxides, zirconium acetate, Zr(CH₂CHCO₂)₄, Zirconium(IV) carbonate,Zr(HPO₄)₂, and Zr(SO₄)₂.

The zirconia precursor may contain impurities and other elements thatare naturally present in the precursor compounds or are unintentionallyintroduced during the zirconia manufacturing process. The group ofpossible impurities and elements includes, but is not limited to thegroup consisting of hafnium and silicon compounds, for example, hafniaand silica.

The zirconia of the invention is a non-modified zirconia. Non-modifiedzirconia is defined as a zirconia or a zirconium-comprising compoundthat has not been modified or doped with another element or compoundduring the methane oxidation catalyst manufacturing process.Non-modified zirconia of this invention does not include sulfatedzirconia or tungsten modified zirconia. As shown in the Examples,modified zirconia (i.e., sulfated or tungsten-modified zirconia) performpoorly in methane oxidation relative to the non-modified zirconiacomprising methane oxidation catalyst of this invention. The presence ofimpurities and other elements that are naturally present in the zirconiaprecursor compounds, such as hafnium, or are unintentionally orintentionally introduced during the zirconia manufacturing process, suchas silicon or silica, as described above, does not make the zirconia amodified zirconia.

During step (a), the zirconia precursor is calcined at a temperature inthe range of from 675 to 1050° C. Although, depending on the nature ofthe precursor used, tetragonal zirconia may be present or will form atcalcination temperatures above 500° C., significant conversion, e.g. oftetragonal zirconia, to monoclinic zirconia may occur at temperatures of675° C. and higher. In some cases, the monoclinic zirconia does not formduring this calcination step, or at least not at detectable levels. Onthe other hand, predominantly to almost pure monoclinic zirconia isobtained upon calcination at very high temperatures of 1100° C. andabove. Preferably, the zirconia precursor is calcined at a temperatureof in the range of from 750 to 1050° C. The catalysts thus obtained werefound to have an improved methane oxidation activity. Even morepreferably, the zirconia precursor is calcined at a temperature in therange of from 800 to 1025° C.

It is preferred to calcine the zirconia precursor for at least 30minutes. The resulting zirconia precursor comprises tetragonal zirconiaand monoclinic zirconia (if any) wherein the weight ratio of tetragonalzirconia to monoclinic zirconia is in the range of greater than 31:1.

The step of calcining the zirconia precursor is preferably performed inan oxygen-comprising atmosphere, preferably air.

In step (b) of the process, the zirconia obtained in step (a) isimpregnated with a noble metal precursor-comprising impregnationsolution. Preferably, the impregnation solution is an aqueous noblemetal precursor-comprising impregnation solution. The noble metalprecursor-comprising impregnation solution may comprise one or morenoble metal precursors, preferably one or more noble metals selectedfrom the group consisting of palladium, platinum, ruthenium, rhodium,osmium and iridium. More preferably, the noble metalprecursor-comprising impregnation solution comprises one or more noblemetals selected from the group consisting of palladium, platinum andrhodium. Alternatively, the impregnation of step (b) comprises two ormore sequential impregnation steps with the same or different noblemetal precursor-comprising impregnation solutions. Using differentimpregnation solutions may allow for an alternative way to achieve theimpregnation of the zirconia with different noble metals. Any noblemetal precursor that is soluble in the impregnation solution may beused. Suitable palladium, platinum and rhodium noble metal precursorsinclude, but are not limited to, Pd(NO₃)₂, Pd(NH₃)₄(CH₃CO₂)₂,Pd(NH₃)₄Cl₂, PdCl₂, Pd(CH₃CO₂)₂, Pd(NH₃)₄(HCO₃)₂, Palladium(II)acetylacetonate, Palladium(II) citrate, Palladium(II) oxalate, K₂PdCl₄,K₂PdCl₆, Pd(NH₃)₂Cl₄, PdO, Pd(NH₃)₂Cl₄, Pt(NH₃)₂Cl₄, H₂Pt(OH)₆, PtBr₂,PtCl₂, PtCl₄, (NH₄)₂PtCl₆, Pt(NH₃)₂Cl₂, Pt(CN)₂, Pt(NO₃)₂, Pt(NO₃)₄,PtO₂, Platinum(II) acetylacetonate, Platinum(II) acetate, Na₂PtCl₆,K₂PtCl₆, H₂PtCl₆, K₂PtCl₄, Platinum(II) citrate, Platinum(II) oxalate,RhCl₃, Rh₄(CO)₁₂, Rh₂O₃, RhBr₃, Rhodium(II) acetylacetonate, Rhodium(II) citrate, Rhodium (II) oxalate and Rh(NO₃)₃.

It may be advantageous for the impregnating solution to comprise atleast one or more noble metal complexing or chelating compounds in amolar ratio of a complexing or chelating compound to noble metal of from1:1 to 5:1. Suitable complexing or chelating compounds include, but arenot limited to citric acid, sorbitol, oxalic acid, tartaric acid, maleicacid, ethylenediaminetetraacetic acid, acetic acid, crown ethers,bipyridine, bipyrimidine, acetylacetone, ethyldiamine, phenanthroline,trisodium citrate, ammonium citrate, lactic acid, pantoic acid,hydroxypyruvic acid, mannitol, glucose, fructose, hydroxybutyric acid,and methyl cellulose. It was found that the addition of such noble metalcomplexing or chelating compounds, and in particular citric acid, to apalladium precursor-comprising impregnation solution may lead to highercatalyst activity for methane oxidation.

Following the impregnation, the wet noble metal-impregnated zirconia isdried in step (c) at a temperature of no more than 120° C. Preferably,the wet noble metal-impregnated zirconia is dried in step (c) for aperiod of at least 1 hour. The dried noble metal-impregnated zirconia issubsequently calcined in step (d) at a temperature in the range of from400 to 650° C., preferably of from 450 to 600° C. Preferably, the driedimpregnated zirconia is calcined in step (d) for a period of at least 1hour. The step of calcining the noble metal-impregnated zirconia ispreferably performed in an oxygen-comprising atmosphere, preferably air.During the calcining in step (d) at least part of the noble metal willbe converted to noble metal oxides.

It may be advantageous, prior to drying and subsequently calcining thestill wet noble metal-impregnated zirconia in steps (c) and (d), toallow the wet noble metal-impregnated zirconia to age for a period of atleast 1 hour, preferably for a period of from 1 to 5 hours. It has beenfound that such an aging step may result in methane oxidation catalysthaving a higher catalyst activity for methane oxidation.

After the calcination, the resulting zirconia in the methane oxidationcatalyst comprises tetragonal zirconia and monoclinic zirconia whereinthe weight ratio of tetragonal zirconia to monoclinic zirconia is in therange of from 1:1 to 31:1, preferably in the range of from 2:1 to 31:1,even more preferably 2:1 to 28:1, still more preferably 5:1 to 28:1,even still more preferably the range from 10:1 to 27.5:1 and yet evenmore preferably in the range of from 15:1 to 25:1, yet still even morepreferably, 15:1 to 23:1. Calcination processes are well known in theart and the selection of the most suitable calcination temperature andcalcination time will depend on the choice of the zirconia precursor.Such a selection of calcination conditions is within the skills of theperson skilled in the art.

Where the weight ratio of tetragonal to monoclinic zirconia is withinthe range provided by the present invention, the exact choice ofzirconia precursor, calcination procedure and temperature, which wereused to come to such a weight ratio are of less significance.

Preferably, a sufficient amount of noble metals is provided during theimpregnation to provide a methane oxidation catalyst comprising in therange of from 0.5 to 15 wt % of noble metal, based on the total combinedweight of noble metal(s) and the total weight of the tm-ZrO₂.Preferably, thus obtained methane oxidation catalyst comprises at least1 wt % of noble metal, based on the total combined weight of noblemetal(s) and the total weight of the tm-ZrO₂.

The methane oxidation catalyst may be prepared in the form of particles,in particular particles with a size in the range of from 0.1 to 500 um.Depending on the type of zirconia precursor used, the zirconia precursormay be shaped into particles of a desired particle size prior to step(a) or alternatively the zirconia obtained from step (a) may be shapedinto particles of a desired particle size. Suitable methods forpreparing the zirconia or zirconia precursor particles of a desiredparticle size include, but are not limited to: wet milling, wetgrinding, dry grinding, mulling, thermal treatment, precipitation orspray drying. Such zirconia particles have a high surface area, allowingfor an improved distribution of the noble metal on the zirconia, whichis beneficial for the methane oxidation activity of the final catalyst.It has been found that some methods to reduce the particle size of thezirconia or zirconia precursor particles may cause a change to thecrystallographic zirconia phase composition, i.e., change in the weightratio of tetragonal to monoclinic zirconia in the zirconia. Inparticular, it has been observed that when using methods such as wetmilling, wet grinding or dry grinding, the tetragonal zirconia that waspresent after the previous calcination at elevated temperatures couldpartially be transformed to monoclinic zirconia. In this case, theweight ratio of tetragonal to monoclinic zirconia in the tm-ZrO₂ couldbe slightly reduced. Without wishing to be bound to any particulartheory it is believed that this further formation of monoclinic zirconiais caused by the occurrence of localized increases in the temperaturecaused by the friction between the particles and/or, in general, by theenergy impact of the milling process on the crystalline tetragonalzirconia structure. This increased temperature may locally result in asmall additional thermal conversion of tetragonal to monocliniczirconia.

The methane oxidation catalyst according to the present invention may beprepared in any suitable form or size, including but not limited to theabove mentioned particles, powders, extrudates, rings, pellets, tablets,or monoliths. The methane oxidation catalyst may be deposited on asubstrate in the form of a layer, film or coating. In one preferredembodiment, the process for preparing the methane oxidation catalystaccording to the invention comprises depositing the noblemetal-impregnated zirconia after calcination in step (d) in the form ofa layer, film or coating on a ceramic or metallic monolith substrate. Inan equally preferred embodiment, the process for preparing the methaneoxidation catalyst according to the invention comprises depositing thezirconia obtained in step (a) in the form of a layer, film or coating ona ceramic or metallic monolith substrate and subsequently impregnatingand treating the zirconia deposited on the monolith substrate accordingto steps (b) to (d). Suitable ceramic or metallic monoliths have beendescribed herein above. The zirconia or impregnated zirconia arepreferably deposited by contacting the monolith with a suspension ofzirconia, that is preferably aqueous or impregnated zirconia particles,in particular particles with a size in the range of from 0.1 to 500 μm.Preferably, the noble metal-impregnated zirconia obtained in step (d) orthe zirconia obtained in step (a) is deposited on the ceramic ormetallic monolith by a washcoating step. Typically, the noblemetal-impregnated zirconia obtained in step (d) or the zirconia obtainedin step (a) are provided in the form of a suspension to the washcoatingstep. In the washcoating step the zirconia or noble metal-impregnatedzirconia particles are suspended in the washcoat suspension prior toapplication to the monolith substrate. Washcoating the substrate resultsin depositing of a thin layer of zirconia particles or noblemetal-impregnated zirconia particles on the surface of the pore channelsof the monolith substrate, which in turn maximizes the catalyticallyactive surface of the catalyst available for the oxidation of methane.Where the noble metal-impregnated zirconia obtained in step (d) or thezirconia obtained in step (a) is deposited on the ceramic or metallicmonolith substrate by a washcoating step, the preferred particle size ofthe noble metal-impregnated zirconia or the zirconia particles in thewashcoat is in the range of from 0.1 to 50 μm, more preferably of from0.1 to 20 μm as determined by light scattering. If the particle size ofthe zirconia is too large, the washcoat suspension may be subjected towet milling in order to reduce the particle size of the zirconia orimpregnated zirconia particles to sizes in the above mentioned range.

In one embodiment, the process for preparing a methane oxidationcatalyst comprises following step (a), but before step (b):

-   -   (i) preparing an aqueous suspension of the zirconia containing        in the range of from 10 to 65 wt % of zirconia based on the        weight of the total suspension;    -   (ii) adding an acid to the aqueous suspension of zirconia        particles to adjust the pH of the suspension to a pH of in the        in the range of from 3 to 6, preferably of from 3.5 to 4.5;    -   (iii) wet milling the aqueous suspension of zirconia particles        until the suspension comprises zirconia particles of an average        particle size by volume of at most 20 μm as determined by light        scattering and optionally readjusting pH of the milled        suspension with an acid to maintain it in the range of from 3 to        6, preferably from 3.5 to 4.5;    -   (iv) washcoating a layer of the suspension obtained in        step (iii) on the surface of a ceramic or metallic monolith        substrate; and    -   (v) drying the washcoated ceramic or metallic monolith substrate        at a temperature of no more than 120° C. for a period of at        least 1 hour.

In the subsequent step (b), the washcoated ceramic or metallic monolithsubstrate, and in particular the zirconia in the washcoat, isimpregnated with the noble metal precursor-comprising impregnationsolution after which it is dried and calcined according to step (c) and(d) to produce the finished methane oxidation catalyst. Optionally, thewashcoated ceramic or metallic monolith substrate of step (v) iscalcined at a temperature in the range of from 400 to 650° C.,preferably of from 450 to 600° C., for a period of at least 1 hour priorto impregnation with the noble metal impregnation solution.

In an alternative embodiment, steps (i) to (v) are performed after step(d) using the noble metal-impregnated zirconia to prepare the aqueoussuspension in step (i). In such a case step (v) is followed by acalcination of the dried washcoated ceramic or metallic monolithsubstrate at a temperature in the range of from 450 to 650° C.,preferably of from 450 to 600° C., for a period of at least 1 hour toproduce the finished methane oxidation catalyst.

The washcoating step may be performed using any suitable washcoatingprocedure, including but not limited to (1) dipping the monolithsubstrate into the suspension, (2) pouring the suspension over themonolith substrate or (3) forcing the suspension through the porechannels of the monolith substrate.

Optionally, in the range of from 5 to 20 wt %, based on the weight ofthe zirconia, of a binder material may be added to the zirconia particlesuspension prior to step (iii). In addition, optionally, in the range offrom 1 to 20 wt %, based on the weight of the zirconia in the suspensionprepared in step (i), of a viscosity modifying compound may be added tothe zirconia particle suspension prior to step (iii). Suitable viscositymodifying compounds include, but are not limited to acetic acid, citricacid, methyl cellulose, chitin, starch, glucose and fructose.

Alumina or zirconia may be used as a binder, but the amount of aluminaor zirconia introduced as a binder in the catalyst should be limited toless than 20 wt %, based on the weight of the methane oxidationcatalyst. If used, the amount of alumina or zirconia should be in therange of from 1 wt % to 20 wt %, preferably in the range of from 2 wt %to 10 wt %.

Preferably, in the range of from 50 to 400 kg, more preferably of from75 to 300 kg, of zirconia, excluding any zirconia added as binder, iswashcoated on the monolith substrate per m³ of monolith substrate.Preferably, during the impregnation with the noble metal comprisingimpregnation solution, an amount of noble metal precursor is absorbed oradsorbed or deposited on the zirconia to obtain a final catalystcomprising in the range of from 1 to 16 kg of noble metal per m³ ofmonolith substrate, more preferably in the range of from 1 to 8 kg ofnoble metal per m³ of monolith substrate.

In another aspect, the invention provides a methane oxidation catalystprepared by a process for preparing a methane oxidation catalystaccording to the present invention.

In a further aspect, the invention provides a method of oxidizingmethane. This method comprises contacting a gas stream comprisingmethane with a methane oxidation catalyst according to the presentinvention, as described herein, in the presence of oxygen and oxidizingat least part of the methane in the gas stream to carbon dioxide andwater. In a certain instance, the gas stream comprising methane is anexhaust gas from a natural gas-fueled engine. The natural gas-fueledengine may be fueled by a fuel comprising atmospheric pressure naturalgas, compressed natural gas, liquefied natural gas, or a combinationthereof. In a particular aspect, the natural gas-fueled engine is fueledby compressed natural gas or liquefied natural gas. The natural gas fuelcould be spark ignited or diesel ignited. Alternatively, the naturalgas-fueled engine is fueled by a mixture of natural gas and one or moreother hydrocarbon fuels, including, but not limited to gasoline,kerosene, diesel or gasoil, in particular a mixture of compressednatural gas or liquefied natural gas with diesel or gasoil. In anotheralternative, the natural gas-fueled engine may be fueled by eithernatural gas or by a hydrocarbon fuel.

Any natural gas-fueled engine is contemplated. Exemplary naturalgas-fueled engines include heavy duty transport engines, such as thoseused in the trucking, mining, marine, and rail industries. Additionalexemplary natural gas-fueled engines include stationary service engines,such as natural gas compressors, gas turbines, and power plant serviceengines. Natural gas-fueled engines may operate alternatively in eitherfuel-lean or fuel-rich burn modes. Fuel-lean burn mode refers to engineoperation in which fuel is burned with an excess of air, i.e. oxygen.For example, in fuel-lean burn mode, oxygen molecules and methanemolecules may be provided to the natural gas-fueled engine in a molarratio of oxygen to methane molecules (also referred to as O₂:CH₄ ratio)up to 100:1. Fuel-rich burn mode, as used herein, means maintaining an,approximately, stoichiometric ratio of oxygen molecules to hydrocarbonmolecules, i.e. an O₂:CH₄ ratio of 2. Preferably, the natural gas-fueledengine is operated in a fuel lean burn mode. By operating the naturalgas-fueled engine in a fuel lean mode, at least part, and preferablyall, of the oxygen required to oxidize the methane in the exhaust gas isprovided as part of the exhaust gas.

The method to oxidize methane according to the present invention may beused with an exhaust gas which contains a methane concentration of lessthan or equal to 10000 ppm by volume (ppmv), preferably in the range offrom 25 ppmv to 10000 ppmv, more preferably of from 50 to 5000 ppmv andeven more preferably from 100 to 3000 ppmv.

Preferably, the methane and oxygen are contacted with the methaneoxidation catalyst in a O₂:CH₄ ratio at least 2:1, more preferably atleast 10:1, even more preferably at least 30:1, still even morepreferably at least 50:1, yet more preferably at least 100:1.Preferably, the methane and oxygen are contacted with the methaneoxidation catalyst in a O₂:CH₄ ratio of in the range of from 2:1 to200:1, more preferably of from 10:1 to 200:1, even more preferably offrom 30:1 to 200:1, still even more preferably of from 50:1 to 200:1,yet more preferably if from 100:1 to 200:1.

Preferably, the methane and oxygen are contacted with the methaneoxidation catalyst at a temperature in the range of from 120 to 650° C.,more preferably of from 250 to 650° C., still more preferably 300 to600° C.

The oxygen used to oxidize the methane may be provided as part of thegas stream comprising methane, for instance the exhaust gas, and/or froman external source, such as air, oxygen enriched air, pure oxygen ormixtures of oxygen with one or more other, preferably inert, gases.Optionally, where part or all of the oxygen is provided from a sourceother than an exhaust gas it may be advantageous to preheat the oxygenprior to contacting the oxygen with the methane.

The gas stream comprising methane may further comprise in the range offrom 0 to 20 vol % water, preferably of from 8 to 15 vol %.

The gas stream which comprises methane may further comprise from 0 to 50ppm by volume of SO₂, preferably of from 0 to 30 ppm by volume of SO₂.Sulphur is known to those skilled in the art for its ability todeactivate noble metal catalysts. To reduce sulphur-based deactivationof the catalyst, the method according to the invention may thereforeinclude contacting the gas stream comprising methane with a SO₂absorbent, prior to contacting the methane oxidation catalyst, to removeat least part of the SO₂ from the gas stream comprising methane.

In a particular application, the step of contacting a stream whichcomprises methane with the methane oxidation catalyst occurs at a streamgas hourly space velocity (GHSV) in the range of from 10,000 to 120,000hr⁻¹, preferably of from 20,000 to 100,000 hr⁻¹.

In a specific application, the method of oxidizing methane results in atleast 50 vol % of the methane in the stream being oxidized at atemperature of equal to or less than 450° C., preferably 405° C., morepreferably 400° C., still more preferably 395° C., even still morepreferably 390° C.

EXAMPLES

The following examples illustrate the present invention but are notintended to limit the scope of the invention.

The following test procedures were used:

Test Procedures

Catalyst Performance Evaluation Test

The catalyst methane oxidation activity measurements were carried out ina fully automated parallelized, high throughput catalyst testing rigwith 48 fixed bed reactors (each reactor with a total volume of 1 mL)made from stainless steel. The catalysts were tested using simulatedexhaust gas compositions and operating conditions similar to those ofnatural gas fueled engines operated with an oxygen surplus (fuel-lean).The conditions used for the testing are shown in Table 1.

TABLE 1 Temperature range 375-550° C. Pressure Ambient Exhaust gas 2000ppmv CH₄, 1000 ppmv CO, 150 ppmv composition NO, 7.5 vol % CO₂, 6 vol %O₂, 15 vol % H₂O, balance N₂ GHSV 50000 hr⁻¹ Mass of catalyst 0.2 gCatalyst particle 315-500 μm size fraction

A sieved fraction of the catalysts with particle sizes from 315-500 μmwas used for the catalytic performance testing. For the reactor loading,the desired catalyst mass was diluted with an inert material (corundum)of the same particle size fraction to a total reactor volume of 1 mL.This was done in order to mimic a methane oxidation catalyst provided ona monolith substrate with a deposition of 200 g of a methane oxidationcatalyst washcoat per liter of catalyst volume (including the monolithsubstrate).

The T₅₀ (CH₄) values (temperature requirement for 50 vol % CH₄conversion after >100 h time-on-stream) were used as criteria for theevaluation of methane oxidation activity. The comparison of methaneoxidation activity was done at equal noble metal loading levels (4 wt %)for all catalysts. The CO conversion during these tests was determinedto be 100% for all tested catalytic materials in the temperature rangedescribed in Table 1.

Quantitative Analysis of Crystalline Tetragonal and Monoclinic ZrO₂Phases Composition

The powder XRD crystallographic phases analysis of zirconia basedcatalytic materials was carried out on a Bruker D8 Advance X-raydiffraction system (Diffrac.EVA software, Bragg-Brentano geometry; highresolution LYNXEYE XE detector; Cu Kα radiation (λ=1.5406 Å) in the 20range from 5° to 140°; 1° steps; scan rate 0.02°/sec; goniometer radius28 cm; Ni filter; applied power 40 kV/40 mA).

The quantitative phase analysis was carried out for diffraction patternsof samples using the TOPAS software package (Version 4.2) available fromBruker company. Reference materials were used for peak identification[tetragonal zirconia (00-050-1089)/monoclinic zirconia(00-037-1484)/palladium oxide (00-041-1107)]. These reference materialsdata are available in the TOPAS software package. Quantitative phaseanalysis was performed using software assisted Rietveld refinement. Therefinement was performed by least-square fitting of a theoretical powderdiffraction pattern against the measured powder diffraction pattern. Thefitting included a Chebychev polynomial fit and a Pearson VII profilefit function while having lattice parameters and crystallite sizes asopen parameters. For each fitted powder diffraction pattern the presenceof tetragonal zirconia, monoclinic zirconia and palladium oxide wasverified. The quantification method resulted in phase content by weightwhich was used for the calculation of phase ratio by weight by dividingthe phase content of tetragonal zirconia by the phase content ofmonoclinic zirconia. Alternatively, the tetragonal to monocliniczirconia phase ratio could be determined as a ratio of signalintensities at 2θ=30.1° characteristic for tetragonal zirconia phase andat 2θ=28.1° characteristic for monoclinic zirconia phase.

Samples and Sample Preparation

Several samples of methane oxidation catalyst were prepared in supportof the present invention.

Sample 1 (t-ZrO₂ Prepared Via Calcination of Amorphous ZirconiumHydroxide)

10 g of a zirconium hydroxide powder with average particle size of 5.5μm, loss of ignition (LOI) as determined in air at 600° C. of 33.2 wt. %and amorphous crystallographic phase structure was calcined in a flow ofair using 5° C./min heating rate to a temperature of 850° C. and held atthis temperature for 12 hours.

Sample 1A (tm-ZrO₂)

For the preparation of the tm-ZrO₂, a 5 g quantity of Sample 1 wasimpregnated with an impregnating solution containing 0.643 mL of a 4.5mol/L aqueous nitric acid solution and 0.607 mL of deionized (furtherreferred to as DI) water. The impregnated wet powder was then stored ina sealed container for 5 hours, dried at 80° C. for 16 hours andsubsequently calcined in a flow of air using 5° C./min heating rate to600° C. and held at this temperature for 12 hours.

Sample 1B (Pd/Tm-ZrO₂ Catalyst)

For the preparation of the Pd/tm-ZrO₂ catalyst, a 4.5 g quantity ofSample 1A was impregnated with 0.477 mL of an aqueous HNO₃-containingPd(NO₃)₂ solution (Pd concentration 3.651 mol/L) diluted with 0.423 mLDI water prior to impregnation to match the pore volume of the zirconia.The impregnated wet sample was then aged for 3 hours in a closedcontainer at room temperature and dried for 16 hours at 80° C. in adrying oven. Subsequently, the prepared catalyst sample was placed in anoven and calcined in a flow of air using 5° C./min heating rate to 600°C. and held at this temperature for 12 hours. The Pd content of thefinal Pd/tm-ZrO₂ catalyst was determined to be 4 wt. %, based on thewhole catalyst sample.

Comparative Sample 4 (t-ZrO₂)

A tetragonal zirconia powder (Saint-Gobain, ID# SZ61152, 3 mm diameter,lot#2005820395) was crushed and sieved to obtain a fraction withparticle sizes in the range of 315-500 μm. The resulting powder was thencalcined in a flow of air using 5° C./min heating rate to 650° C. andheld at this temperature for 12 hours.

Comparative Sample 4A (Pd/t-ZrO₂ Catalyst)

A methane oxidation catalyst was prepared using 3 g of the zirconia ofSample 4, which was impregnated with 1.164 mL of an aqueousHNO₃-containing Pd(NO₃)₂ solution (Pd concentration 1 mol/L) which wasdiluted with 0.636 mL of DI water prior to impregnation. The impregnatedsample was aged for 3 hours in a closed container at room temperatureand dried for 16 hours at 80° C. in a drying oven. Subsequently, thecatalyst sample was placed in an oven and calcined in a flow of airusing 5° C./min heating rate to 600° C. and held at this temperature for12 hours. The Pd content of the final catalyst sample was determined tobe 4 wt. %, based on the whole catalyst sample.

Comparative Sample 5 (m-ZrO₂)

A monoclinic zirconia powder (Saint-Gobain, ID# SZ31164, 3.175 mmdiameter, lot# SN2004910029) was crushed and sieved to obtain a fractionwith particle sizes in the range of 315-500 μm. The resulting powder wascalcined in a flow of air using 5° C./min heating rate to 650° C. andholding at this temperature for 12 hours.

Comparative Sample 5A (Pd/m-ZrO₂ Catalyst)

An amount of 3 g of Comparative Sample 5 was impregnated with 1.162 mLof an aqueous HNO₃-containing Pd(NO₃)₂ solution (Pd concentration 1mol/L) which was diluted with 1.238 mL of DI water prior toimpregnation. The impregnated sample was then aged for 3 hours in aclosed container at room temperature and dried for 16 hours at 80° C. ina drying oven. Subsequently, the catalyst sample was placed in an ovenand calcined in a flow of air using 5° C./min heating rate to 600° C.and held at this temperature for 12 hours. The Pd content of the finalcatalyst sample was determined to be 4 wt. %, based on the wholecatalyst sample.

Comparative Sample 6 (gamma-Al₂O₃)

An alumina extrudate sample (Saint-Gobain, ID#SA 6175, 1.59 mm diameter,lot#9608006) was crushed and sieved in order to obtain a fraction withparticle sizes in the range of 315-500 μm. The resulting powder was thencalcined in air at 650° C. for 12 h.

Comparative Sample 6A (Pd/Al₂O₃ Catalyst)

An amount of 50 g of the gamma-alumina of Sample 6 was impregnated witha solution of 18.78 mL aqueous HNO₃-containing Pd(NO₃)₂ (conc. 1 mol/L)which was diluted with 31.22 mL of DI water prior to impregnation. Theimpregnated wet catalyst sample was then aged for 3 hours in a closedcontainer at room temperature and dried for 16 hours at 80° C. in adrying oven. Subsequently, the dried catalyst sample was calcined in aflow of air by heating it at 5° C./min to 600° C. and holding at thistemperature for 12 hours. The Pd content of the obtained catalyst samplewas found to be 4 wt. %, based on the whole catalyst sample.

Comparative Sample 7 (PdPt/Gamma-Al₂O₃ Catalyst)

An alumina extrudate sample (Saint-Gobain, ID# SA 6175, 1.59 mmdiameter, lot#9608006) was crushed and sieved in order to obtain afraction with particle sizes on the range of 315-500 μm. The resultingpowder was calcined in air at 650° C. for 12 h. For the impregnationwith noble metal, 3 g of the alumina fraction was impregnated with asolution containing 0.97 mL aqueous HNO₃-containing Pd(NO₃)₂ (conc. 1mol/L) and 0.22 mL aqueous HNO₃-containing Pt(NO₃)₂ (conc. 0.5 mol/L)which was diluted with 0.62 mL of DI water prior to impregnation. Theobtained wet catalyst sample was aged for 3 hours in a closed containerat room temperature and dried for 16 hours at 80° C. in a drying oven.Subsequently, the catalyst was calcined in a flow of air using 5° C./minheating rate to 600° C. and held at this temperature for 12 hours. Thetotal (Pd and Pt) noble metal content of the final catalyst was found tobe 4 wt. %; 3.6 wt. % Pd and 0.4 wt. % Pt, based on the whole catalystsample.

Comparative Sample 8 (Physically Mixed t-ZrO₂ and m-ZrO₂)

A tetragonal zirconia powder (Saint-Gobain, ID# SZ61152, 3 mm diameter,lot#2005820395) was crushed and sieved to obtain a fraction withparticle sizes from 315-500 μm. The resulting powder was then calcinedin a flow of air using 5° C./min heating rate to 650° C. and holding atthis temperature for 12 hours to obtain zirconia that shows only XRDreflections belonging to t-ZrO₂ (Zirconia powder A). A monocliniczirconia powder (Saint-Gobain, ID# SZ31164, 3.175 mm diameter, lot#SN2004910029) was crushed and sieved to obtain a fraction with particlesizes from 315-500 μm. The resulting powder was calcined in a flow ofair using 5° C./min heating rate to 650° C. and holding at thistemperature for 12 hours to obtain a zirconia which exhibited powder XRDreflections belonging to pure m-ZrO₂ (zirconia powder B). Both zirconiapowders were mixed to prepare a physical mixture containing 95 wt. %(t-ZrO₂, zirconia powder A) and 5 wt. % (m-ZrO₂, zirconia powder B)ratio, equivalent to a tetragonal to monoclinic weight ratio of 19:1.

Comparative Sample 8A (Pd/Physical Mixture t-ZrO₂/m-ZrO₂ Catalyst)

An amount of 1.5 g of the physically mixed t/m-ZrO₂ powder ofComparative Sample 7 was impregnated with 0.582 mL of an aqueousHNO₃-containing Pd(NO₃)₂ solution (Pd concentration 1 mol/L). Prior toimpregnation, the above Pd(NO₃)₂ solution was diluted with 0.093 mL ofDI water. The wet catalyst sample was then aged for 3 hours in a closedcontainer at room temperature and then dried for 16 hours at 80° C. in adrying oven. Subsequently, the dried catalyst sample was placed in anoven and calcined in a flow of air using 5° C./min heating rate to 600°C. and holding at this temperature for 12 hours. The Pd content of thefinal catalyst was determined to be 4 wt. %, based on the whole catalystsample invention.

Comparative Sample 9 (t-ZrO₂-S)

A tetragonal, sulfur-modified zirconia powder (Saint-Gobain, ID#SZ61192, 3 mm diameter, lot#2013820069) was crushed and sieved to obtaina fraction with particle sizes in the range of 315-500 μm. The resultingpowder was calcined in a flow of air using 5° C./min heating rate to650° C. and holding at this temperature for 12 hours.

Comparative Sample 9A (NU t-ZrO₂-S Catalyst)

An amount of 3 g of Comparative Sample 9 was impregnated with 1.163 mLof an aqueous HNO₃ containing Pd(NO₃)₂ solution (Pd concentration 1mol/L) which was diluted with 0.036 mL of DI water prior toimpregnation. The impregnated t-ZrO₂-S sample was then aged for 3 hoursin a closed container at room temperature and dried for 16 hours at 80°C. in a drying oven. Subsequently, the catalyst sample was placed in anoven and calcined in a flow of air using 5° C./min heating rate to 600°C. and held at this temperature for 12 hours. The Pd content of thefinal PD/t-ZrO₂-S catalyst sample was determined to be 4 wt. %, based onthe whole catalyst sample.

Comparative Sample 10 (t-ZrO₂-W)

A tetragonal, tungsten-modified zirconia powder (Saint-Gobain, ID#SZ61143, 3 mm diameter, lot#2014820006) was crushed and sieved to obtaina fraction with particle sizes in the range of 315-500 μm. The resultingpowder was calcined in a flow of air using 5° C./min heating rate to650° C. and holding at this temperature for 12 hours.

Comparative Sample 10A (Pd/t-ZrO₂-W Catalyst)

An amount of 3 g of Comparative Sample 10 was impregnated with 0.580 mLof an aqueous HNO₃ containing Pd(NO₃)₂ solution (Pd concentration 1mol/L) which was diluted with 0.560 mL of DI water prior toimpregnation. The impregnated sample was then aged for 3 hours in aclosed container at room temperature and dried for 16 hours at 80° C. ina drying oven. Subsequently, the sample was impregnated for a secondtime with 0.580 mL of an aqueous HNO₃ containing Pd(NO₃)₂ solution (Pdconcentration 1 mol/L) which was diluted with 0.560 mL of DI water priorto impregnation. The impregnated sample was then aged for 3 hours in aclosed container at room temperature and dried for 16 hours at 80° C. ina drying oven. Finally, the catalyst sample was placed in an oven andcalcined in a flow of air using 5° C./min heating rate to 600° C. andheld at this temperature for 12 hours. The Pd content of the finalPd/t-ZrO₂-W catalyst sample was determined to be 4 wt. %, based on thewhole catalyst sample.

Results

The analysis of the powder XRD patterns for the different samples ofthis invention and these of prior art resulted in the determination ofthe following tetragonal and monoclinic crystallographic phases ratios(see Table 2):

TABLE 2 Tetragonal/monoclinic Sample ZrO₂ weight ratio 1A 11.2:1 4A37.2:1 5A 100% m-ZrO₂

Table 3 shows the T₅₀(CH₄) values of the zirconia based catalyst Samples1B and comparative catalyst Samples 4A and SA. Table 3 further shows theT₅₀(CH₄) value for alumina based comparative catalyst Sample 6A.

TABLE 3 sample T₅₀(CH₄) [° C.] 1B  401 4A* 404 5A* 413 6A* 479*Comparative Samples

1. A process for preparing a methane oxidation catalyst comprising thefollowing steps: a.) calcining a non-modified zirconia precursor at atemperature in the range of from 675 to 1050° C. to prepare tetragonalzirconia wherein the weight ratio of tetragonal zirconia to monocliniczirconia, if any is present, is greater than 31:1; b.) impregnating thezirconia obtained from step b.) with a noble metal precursor-comprisingimpregnation solution; c.) drying the wet noble metal-impregnatedzirconia at a temperature of no more than 120° C.; and d.) calcining thedried noble metal-impregnated zirconia at a temperature in the range offrom 400 to 650° C. to prepare a methane oxidation catalyst wherein theweight ratio of tetragonal zirconia to monoclinic zirconia of thecatalyst is in the range of from 1:1 to 31:1.
 2. The process of claim 1,wherein the weight ratio of tetragonal zirconia to monoclinic zirconiaof the calcined zirconia precursor in step a.) is in the range of from35:1 to 1000:1.
 3. The process of claim 1 wherein no detectablemonoclinic zirconia is present after the calcination of step a.).
 4. Theprocess of claim 1, further comprising depositing the noblemetal-impregnated zirconia after calcination in step (d) in the form ofa layer, film or coating on a ceramic or metallic monolith substrate. 5.The process of claim 1, further comprising depositing the zirconiaobtained in step (a) in the form of a layer, film or coating on aceramic or metallic monolith substrate and subsequently impregnating andtreating the deposited zirconia according to steps (b) to (d).
 6. Theprocess of claim 1, wherein the impregnated zirconia obtained in step(d) or the zirconia obtained in step (a) is deposited on the ceramic ormetallic monolith by a washcoating step.
 7. The process of claim 1,wherein the impregnation solution comprises a noble metal precursorselected from the group consisting of palladium compounds, platinumcompounds, rhodium compounds and mixtures thereof.
 8. The process ofclaim 1, wherein the impregnation solution comprises at least one ormore noble metal complexing or chelating compounds in a molar ratio ofthe complexing or chelating compounds to noble metal of from 1:1 to 5:1.9. The process of claim 1, wherein the calcination in step a.) iscarried out at a temperature in the range of from 800 to 1025° C. 10.The process of claim 1, wherein the non-modified zirconia precursorcomprises tetragonal zirconia.
 11. The process of claim 1, wherein thenon-modified zirconia precursor is not sulfated and nottungsten-modified.
 12. The process of claim 1, wherein the monocliniczirconia is present as a dispersion of monoclinic zirconia in thetetragonal zirconia.
 13. The process of claim 4, wherein the zirconia isdeposited on a ceramic or metallic monolith substrate comprising porechannels, defining an inner pore channel surface, and wherein thezirconia is deposited in the form of a coating, washcoat or a film of athickness in the range of from 10 to 250 μm on the inner pore channelsurface.
 14. The process of claim 1, wherein the dried noblemetal-impregnated zirconia comprises in the range of from 0.5 to 15 wt %of total noble metals, based on the total weight of the noble metal(s)and tetragonal and monoclinic zirconia
 15. A methane oxidation catalystprepared by a process claim
 1. 16. A method of oxidizing methane bycontacting a gas stream comprising methane with a methane oxidationcatalyst according to claim 15 in the presence of oxygen and oxidizingat least part of the methane in the gas stream to carbon dioxide andwater.
 17. A method according to claim 16, wherein the stream comprisingmethane is an exhaust gas from a natural gas-fueled engine.